khagol Mandali

Hello! This Wednesday, we discussed a very simple looking question...Why the sky is blue? The answer to this question comes in one word as "Scattering of light"! But what exactly this scattering is and what is a thing scattering the light? Well, when we talk about sky color we are essentially talking about Rayleigh scattering!

Rayleigh scattering is scattering of light due to particles much smaller than wavelength of light. It occurs when light travels in transparent solids and liquids, but is most prominently seen in gases. The intensity of scattered light varies as inverse fourth power of wavelength of light. So for shorter wavelengths , scattering is more! In the visible spectrum, red light has a wavelength of 720 nm whereas blue has 380 nm. So the if we calculate, the blue light is scattered almost 10 times more than red light and this is a reason why our sky is blue.

Well, our eye structure also plays an important role in the sky looking blue. There are 3 types of receptors in our eyes - Red(R),Green(G) and Blue(B). Out of these 3, B receptors respond more strongly as compared to R and G.

Now, one common misconception is this scattering is due to dust particles! But that's not the case...had it been due to dust particles or droplets of water, the sky color would have changed every now and then as the humidity or haze conditions changed! It was later proved by Einstein that the oxygen and nitrogen molecules produce this Rayleigh scattering.

And then, if smaller the wavelength of light, more the scattering then sky should look violet but this isn't the case! The reason is the violet light scattered in a high amount, is absorbed in high atmosphere only so what reaches earth is the blue light only!

Now the obvious question comes...why the sky is red at sunrise and sunset? The reason again lies in scattering of light! At Sunrise or Sunset, the Sun is at horizon and hence, the light rays travel greater distance. In between blue color is lost and what we see is a red light and similar shades!

In the end, we come down to a very basic question -
What is exactly happening in "scattering" or why molecules "scatter" the light? Well, the molecules scatter light because the electromagnetic field of light waves induces electric dipole moments in the molecules.

Going along the same line, we can find out why clouds are white and why fire looks red? Any guesses?